Monopole antennas are widely used in various applications, particularly in mobile wireless communications because they are simple to construct, compact, robust and easy to install and change when required. These properties together with the omni-directional radiation pattern make monopole antennas ideal candidates for many consumer products such as mobile phones, pagers, remote control toys, etc. In order to meet the demand of future emerging broadband wireless services, it is necessary to improve the monopole's bandwidth characteristic, while maintaining their desirable properties. Several techniques have been disclosed for monopole bandwidth enhancement. The common feature of these designs is the use of a flat monopole configuration, which affects the pattern uniformity in the horizontal plane. P. V. Anob and G. Kumar, in a paper entitled Wide-band modified triangular monopole antennas, Proc. Of the 8th Int. Symp. On Microwave and Optical Tech., ISMOT 2001, Montreal, Canada, June 2001, pp. 169-172 disclose the use of two orthogonal flat monopoles to improve the horizontal plane pattern. However this approach results in an undesirably volumetrically large monopole.
It is known to excite a dielectric resonator antenna (DRA) using a probe sometimes referred to as a monopole. Notwithstanding, the probe is typically used solely to excite the fields within the DRA and does not act itself as a radiating element; in these instances, only the DRA is responsible for radiation. This is evident in the radiation patterns, which do not display the characteristic pattern of a monopole antenna, with a pattern null in the direction of the probe's vertical axis, but that of a DRA, which typically has a maximum in the vertical direction. A condition for which these probes do not radiate is when their physical height is significantly less than a quarter wavelength of the operating frequency. Consistent with this, the probes used to excite the DRAs are less than one eighth of a wavelength. Such an antenna is described in U.S. Pat. No. 5,940,036 in the name of Oliver et al., entitled Broadband Circularly Polarized DRA and is also described in a paper entitled General Solution of a Monopole Loaded by a Dielectric Hemisphere for Efficient Computation, K. W. Leung, IEEE Trans. AP, Vol. 48, No. 8, August 2000, pp. 1267-68. These references do not disclose a broadband monopole maintaining a desirable circulatory symmetrical configuration for a uniform horizontal coverage pattern. They disclose a DRA with a monopole probe feed having an output response of a DRA, which is different than that of the monopole.
In a paper entitled Stacked annular Ring Dielectric Resonator Antenna Excited by Axi-Symmetric Coaxial Probe, by S. M. Shum and K. M. Luk, IEEE Trans. AP Vol. 43, No. 9, August 95, pp. 889-892, two annular-ring DRAs are arranged in a vertically stacked configuration where the lower DRA is fed with a short probe and small air gaps are introduced between the two DRAs. The addition of the upper DRA improves the impedance bandwidth from 11.5% to 18%. but again the probe is less than an eighth of a wavelength and does not contribute to the radiation.
Japanese Patent Application No. 08149368 filed Nov. 6, 1996 in the names of Kawabata Kazuya et al., assigned to Murata Mfg. Co. Ltd., discloses a monopole antenna shown loaded with a plurality of dielectric layers forming a dielectric element. The dielectric element is said to cover the monopole and is shown to do so. In this configuration, the monopole antenna would be radiating, and the dielectric layers are used to assist in shaping the radiation pattern. These dielectric layers are located significantly above the ground plane and are thus not behaving as a DRA, which is typically placed right against or very near the ground plane separated from the ground plane by a small air gap. Although this invention appears to perform its intended function it does not appear to provide a monopole antenna with a significantly increased bandwidth.
The technique of coating monopole antennas with dielectric material to reduce the resonant frequency of the monopole antenna is well established. In this configuration, the presence of a dielectric coating material simply acts to load the monopole antenna in order to lower the resonant frequency. This allows for a shorter monopole to be used at a given frequency. The dielectric material itself does not radiate within the desired operating frequency range. The condition for radiation can be determined by applying the appropriate equations to determine the resonant frequency of a DRA given the relative permittivity and dimensions of the material.
U.S. Pat. No. 6,147,647, discloses a combination DRA, helix and monopole antenna for multi-band operation. The DRA exited in the HEM mode, behaves like a short horizontal magnetic dipole, which operates independently of the monopole antenna. The DRA produces circular polarized radiation, and the monopole produces linear radiation. The radiation patterns of the monopole and the DRA are also very distinct, with the DRA having maximum radiation in the broadside direction, while the monopole has a null at broadside. In this configuration, the DRA and monopole are specifically designed to minimize any electromagnetic interaction between them and can be treated as two independent antennas. The monopole and DRA have distinct feeds exciting each antenna.
Surprisingly, the antenna in accordance with this invention, provides a synergistic output response which radiates a broadband signal, being significantly broader than the composite output of a monopole and DRA alone, uncoupled.
In the configuration in accordance with this invention, the DRA and the monopole are designed to act in concert. The monopole antenna is excited with a feed, and the monopole antenna itself serves as a feed for the DRA. By exciting the DRA near its centre, the mode (TM01δ) generated within the DRA causes the DRA to radiate the same shape pattern as the monopole. There is a very strong interaction between the monopole and DRA. A novel feature of this invention, is that the dimensions of the monopole and the DRA are selected so that the combination of the two antennas will radiate basically the same pattern over an ultra-wide range of frequencies. The DRA is capable of operating in a TMONδ mode, where N is an integer greater than or equal to 1.
Recently, the Federal Communications Commission (FCC) has allocated 7.5 GHz of spectrum for unlicensed use of ultra-wideband devices (UWB) in the 3.1 to 10.6 GHz frequency band. The UWB spectrum will allow for low-cost, low-complexity, lower power consumption, and high-data-rate wireless connections among devices related to personal wireless communications which are carried, worn, or located near the body (such as wearable computers, a wireless desktop, or a home networking system). These devices will require compact, low-cost, low gain, ultra-wideband antennas, such as the ultra-wideband monopole-DRA in accordance with this invention.
It is an object of this invention to provide a compact broadband monopole while maintaining its desirable circulatory symmetrical configuration for a uniform horizontal coverage pattern.